Powdered epoxy resin compositions are widely used to provide protective coatings. For uses requiring good electrical insulation, aromatic carboxylic acid anhydrides are favored as the curing agents for the epoxy resin. In order to provide reasonably fast curing rates, the epoxy resin and anhydride are usually blended with a catalyst such as an imidazole, tertiary amine, or a metal salt such as stannous octoate. Powders of these catalyzed blends can be stored at ordinary room temperatures but may gradually advance to the point of no longer being usable after a month or two, especially if the temperature goes much above 25.degree. C. for any appreciable portion of the time. Where longer shelf life is required, the catalyst could be omitted, but this would usually result in uneconomically slow curing rates. This problem has been solved by refrigerating the powder, usually at about 5.degree. to 10.degree. C.
Of the anhydrides, trimellitic anhydride is most widely used for electrical insulating uses, being of low cost and providing a more rapid curing of epoxy resins than do other anhydrides. On the other hand, epoxy resin powders employing trimellitic anhydride as the curing agent have been somewhat less shelf-stable than are those which contain other anhydrides, possibly due to its free carboxylic acid group. Trimellitic anhydride is somewhat toxic so that powders containing it should be used only in coating apparatus which has protective controls. U.S. Pat. Nos. 3,269,974 (Childs), 3,384,610 (Lee), 3,400,098 (Parry) and 3,477,971 (Allen et al.) disclose powder comprising a blend of epoxy resin, trimellitic anhydride and, optionally, a catalyst.
U.S. Pat. No. 4,009,224 (Warnken) teaches that coatings of extraordinarily good flexibility can be attained by a blend of epoxy resin, a curing agent, and a copolymer of vinyl acetate and ethylene, but the copolymer tends to reduce shelf-stability, apparently due to free carboxylic acid groups generated upon thermal degradation. Accordingly, refrigeration would be desirable, especially when the curing agent is trimellitic anhydride.
The epoxy resin predominantly used in the aforementioned powdered epoxy resin compositions is a polyglycidyl ether of bisphenol A. It should be high-softening, i.e., have a high Durrans' softening point within the range of 60.degree. to 130.degree. C., preferably within 70.degree. to 110.degree. C. Below 70.degree. C., the powder may tend to sinter during storage unless refrigerated, regardless of the chemical stability of the composition. The Durrans' softening point of the epoxy resin preferably does not exceed 110.degree. C., so as to minimize energy requirements for applying coatings of the powders.